Inorg. Chem. 2007, 46, 5744−5753
Photoinduced Electron-Transfer Processes Based on Novel Bipyridine−Ru(II) Complex: Properties of cis-[Ru(2,2′-bipyridine)2(5,6-bis(3-amidopyridine)-7-oxanorbornene)](PF6)2 and cis-[Ru(2,2′-bipyridine)2(3-aminopyridine)2](PF6)2 Complexes Simone D. Inglez,† Francisco C. A. Lima,† Albe´rico B. F. Silva,† Andreza R. Simioni,‡ Antoˆnio C. Tedesco,‡ Juliana F. S. Daniel,§ Benedito S. Lima-Neto,*,† and Rose M. Carlos*,§ Instituto de Quı´mica de Sa˜ o Carlos, UniVersidade de Sa˜ o Paulo, CP 780, 13650-970 Sa˜ o Carlos, SP, Brasil, Faculdade de Filosofia Cieˆ ncias e Letras de Ribeira˜ o Preto, UniVersidade de Sa˜ o Paulo, Ribeira˜ o Preto, SP, Brasil, and Departamento de Quı´mica, UniVersidade Federal de Sa˜ o Carlos, CP 676, 13565-905 Sa˜ o Carlos, SP, Brasil Received December 27, 2006
This paper presents the synthesis, MO calculations, and photochemical and photophysical properties of cis-[Ru(bpy)2(3Amdpy2oxaNBE)](PF6)2 (2), where bpy is 2,2′-bipyridine and 3Amdpy2oxaNBE is the novel 5,6-bis(3amidopyridine)-7-oxanorbornene chelate-ligand (1). Complex 2 is considered in relation to the cis-[Ru(bpy)2(3Amnpy)2](PF6)2 (3) analogous complex, where 3Amnpy is 3-aminopyridine. Complexes 2 and 3 exhibit absorptions near 350 nm and in the 420−500 nm region attributable to a contribution from MLCT transitions (dπ f bpy and dπ f L; L ) 3Amdpy2oxaNBE or 3Amnpy). Whereas complex 3 is photochemically reactive, complex 2 shows luminescence either at 77 K or at room temperature in fluid solution. The emission of 2 assignable as an MLCT (Ru f bpy) emission is characterized by a long lifetime at room temperature (650 ns in CH3CN and 509 ns in H2O). It is independent of λirr, but it is temperature dependent; i.e., it increases as the temperature is lowered. Considering the chelate ring of 1 contributes to the stability of the complex 2 under continuous light irradiation, the difference in the primary photoprocesses of 3 (loss of 3Amnpy) and 2 (luminescence) may be caused by a lowering of the lowest excited state from 3 to 2. The surface crossing to the lowest MC state value of 987 cm-1 (similar to that of [Ru(bpy)3]2+) will be prevented in the case of complex 2, and as a result, efficient 3Amdpy moiety loss cannot occur. The electronic depopulation of the {Ru(bpy)2} unit and population of a bpy* orbital upon excitation are evident by comparing the photophysical properties with those of a [Ru(bpy)3]2+ related complex. Moreover, a reduction of a bpy ligand in the MLCT excited state is indicated by time-resolved spectra that show features typical of bpy•-. The photocatalytic property of 2 is spectroscopically demonstrated by oxidative quenching using either methylviologen2+ or [RuCl(NH3)5]+2 electron-acceptor ions.
Introduction The design and synthesis of electron donor- and acceptorphotosensitizer arrays have attracted considerable attention as they have potential application in photoelectric molecular devices, catalysts, and biomimetic chemistry.1-3 In particular, * To whom correspondence should be addressed. E-mail: benedito@ iqsc.usp.br (B.S.L.-N.);
[email protected] (R.M.C.). † Instituto de Quı´mica de Sa ˜ o Carlos. ‡ Faculdade de Filosofia Cie ˆ ncias e Letras de Ribeira˜o Preto. § Universidade Federal de Sa ˜ o Carlos. (1) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348-3391.
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by combining inorganic photochemistry with conducting polymers, it is possible to create “molecular wires” that interconnect multiple chromophores and quenchers within a (2) (a) Loiseau, F.; Marzanni, G.; Quici, S.; Indelli, M. T.; Campagna, S. Chem. Commun. 2003, 286-287. (b) Abrahamsson, M. L. A.; Baudin, H. B.; Tran. A.; Philouse, C.; Berg, K. E.; Raymond-Johansson, M. K.; Sun, L.; Akermark, B.; Styring, S.; Hammarstrom, L. Inorg. Chem. 2002, 41, 1534-1544. (3) (a) Xu, Y. H.; Eilers, G.; Borgstrom, M.; Pan, J. X.; Abrahamsson, M.; Magnuson, A.; Lomoth, R.; Bergquist, J.; Polivka, T.; Sun, L. C.; Sundstrom, V.; Styring, S.; Hammarstrom, L.; Akermark, B. ChemsEur. J. 2005, 11, 7305-7314. (b) Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.; Venturi, M. Acc. Chem. Res. 2001, 34, 445-455.
10.1021/ic062478v CCC: $37.00
© 2007 American Chemical Society Published on Web 06/13/2007
Photoinduced Electron-Transfer Processes
single molecular framework.4 A strategy is to develop ligands connected to cyclic olefin which is subjected to ring opening metathesis polymerization (ROMP).5 We have examined olefin-polypyridyl Ru(II)-based compounds that could generate molecular wires, where the monomer-ligand 5,6-bis(3-amidopyridine)-7-oxanorbornene (1; 3Amdpy2oxaNBE), synthesized from the reaction between 3-aminopyridine (3Amnpy) and hydrolyzed exo-7oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride (oxaNBE), reacts with cis-[RuCl2(bpy)2] affording the cis[Ru(bpy)2(3Amdpy2oxaNBE)](PF6)2 complex (2). The optical absorption, characterization, and photochemical and photophysical properties of this novel complex and the related cis[Ru(bpy)2(3Amnpy)2](PF6)2 complex (3) were studied. The results were examined to discover like and unlike characteristics between 2 and 3, observing the benefits of the 7-oxanorbornene moiety in the derivatized monomer-ligand. The results were also examined in relation to the [Ru(bpy)3](PF6)2 complex, and MO calculations of 2 were performed. The choice of 2,2′-bipyridine complexes of Ru(II) is attractive since the related [Ru(bpy)3]2+ complex ion is the most well-known and thoroughly studied photosensitizer.6,7 Because of its octahedral coordination geometry, its derivatives have been extensively investigated, where a specific ligand can be introduced in a controlled manner to tune the photophysical,photochemical,andelectrochemicalproperties.1-4,8 Moreover, many ruthenium complexes connected to norbornene derivatives reported to date5 contain pyridine-based ligands as bridge. Experimental Section General. All reactions were performed under argon atmosphere unless otherwise stated. All chemicals used were of reagent grade or comparable purity. RuCl3_xH2O and [Ru(bpy)3]Cl2_6H2O from Strem; exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride from Aldrich; 2,2′-bipyridine (bpy), 3-aminopyridine (3Amnpy), lithium chloride, sucrose, and ammonium hexafluorophosphate from Aldrich; tetrabutylammonium hexafluorophosphate (TBAPF6) from Fluka; and thionyl chloride (SOCl2) from Merck were used as acquired. HPLC grade acetonitrile and dichloromethane were distilled prior to use. cis-[RuCl2(bpy)2] 2H2O (Anal. Calcd for RuC20H16N4Cl2‚2H2O: C, 46.14; H, 3.84; N, 10.76. Found: C, (4) (a) Pautzsch, T.; Blankenburg, L.; Klemm, E. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 722-732. (b) Barigelletti, F.; Flamigni, L. Chem. Soc. ReV. 2000, 29, 1-12. (c) James, D. K.; Tour, J. M. Molecular Wires and Electronics; Wu¨rthner, F. Ed.; Topics in Current Chemistry 257; Springer-Verlarg: Berlin, 2005; pp 33-62. (5) (a) Rezvani, A.; Bazzi, H. S.; Chen, B. Z.; Rakotondradany, F.; Sleiman, H. F. Inorg. Chem. 2004, 43, 5112-5119. (b) Chen, B. Z.; Sleiman, H. F. Macromolecules 2004, 37, 5866-5872. (c) Carlise, J. R.; Weck, M. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 29732984. (d) Chen, B. Z.; Metera, K.; Sleiman, H. F. Macromolecules 2005, 38, 1084-1090. (6) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277. (7) Kalyanasundaram, K. Photochemistry of Polypiridyne and Porphyrin Complexes; Academic Press: London, 1992. (8) (a) Wu, X.; Fraser, C. L. Macromolecules 2000, 33, 4053-4060. (b) Wu, X.; Fraser, C. L. Macromolecules 2000, 33, 7776-7785. (c) Smith, A. P.; Fraser, C. L. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 4250-4255. (d) Smith, A. P.; Fraser, C. L. Macromolecules 2003, 36, 5520-5525. (e) Fitz-Gerald, J. M.; Jennings, G.; Johnson, R.; Fraser. C. L. Appl. Phys. A 2005, 80, 1109-1113.
46.37; H, 3.39; N, 10.71%) and cis-[Ru(bpy)2(3Amnpy)2](PF6)2‚ 1H2O (3; Anal. Calcd for RuC30H28N8P2F12‚1H2O: C, 39.59; H, 3.22; N, 12.17. Found: C, 39.61; H, 3.30; N, 12.31%) were prepared according to the literature methods.9,10 Synthesis of 5,6-Bis(3-amidopyridine)-7-oxanorbornene (1; 3Amdpy2oxaNBE). The exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3dicarboxylic anhydrid compound (1.0 g, 6.0 mmol) was quantitatively hydrolyzed in 5 mL of water and evaporated to dryness. The isolated monomer (1.0 g; 5.4 mmol) was suspended in 5 mL of dichloromethane and mixed with redistilled SOCl2 (1.0 mL; 14 mmol). The resulting mixture was stirred for 2 h under reflux, and, thus, evaporated to dryness. The solid was washed with anhydrous benzene and dichloromethane and dried under vacuum (30% yield). 13C NMR (CD CN), δ ppm: 50.15, 83.41, 137.93, 170.0. A 0.25 3 g portion of the obtained compound (1.1 mmol) was dissolved in 10 mL of acetone, and 3-aminopyridine (0.21 g; 3.3 mmol) dissolved in 5 mL of acetone was added. The mixture was then refluxed for 2 h and cooled to room temperature, and the precipitate was filtered, washed with acetone, and dried under vacuum (88% yield). 1H NMR (DMSO-d6), δ ppm: 6.53 (q, 2H), 7.49 (dd, 2H, J1 ) 4.7 Hz, J2 ) 8.28 Hz), 8.16 (dd, 2H, J1 ) 2.3 Hz, J2 ) 8.28 Hz), 8.41 (dd, 2H, J1 ) 1.4 Hz, J2 ) 4.7 Hz), 8.88 (d, 2H, J ) 2.4 Hz). 13C NMR (DMSO-d6), δ ppm: 47.08, 79.34, 79.37, 123.77, 126.56, 131.77, 136.80, 141.13, 144.80, 172.60. Anal. Calcd for C18H16O3N4‚3H2O: C, 55.07; H, 5.64; N, 14.33. Found: C, 56.06; H, 5.53; N, 14.52%. Synthesis of cis-[Ru(bpy)2(3Amdpy2oxaNBE)](PF6)2 (2). cis[RuCl2(bpy)2]_2H2O (0.20 g; 0.38 mmol) was dissolved in a 1:1 EtOH/H2O mixture (10 mL), and an amount of 1 (0.13 g; 0.38 mmol) was added. The solution was stirred under argon atmosphere for 8 h under reflux. The volume was reduced to approximately 1 mL, and a stoichiometric amount of NH4PF6 was added to precipitate the complexes. A reddish-brown precipitate that formed for cis-[Ru(bpy)2(3Amdpy2oxaNBE)](PF6)2 (2) was filtered, washed with water and diethyl ether, and dried under vacuum (55% yield). 1H NMR (DMSO-d ), δ ppm: 3.45 (q, 2H, J ) 7.02 Hz, J ) 14 6 1 2 Hz), 4.16 (q, 2H, J1 ) 7.18 Hz, J2 ) 14 Hz), 4.44 (2H), 6.40 (q, 2H), 7.13 (m, 2H), 7.24 (d, 1H, J ) 6.9 Hz), 7.73 (d, 1H, J ) 6.0 Hz), 7.79 (dd, 4H, J1 ) 4.9 Hz, J2 ) 8 Hz), 8.05 (dd, 2H, J1 ) 1.3 Hz, J2 ) 8 Hz), 8.32 (m, 2H), 8.38 (m, 1H), 8.47 (m, 1H), 8.75 (m, 2H), 8.97 (d, 2H, J ) 6.47 Hz), 9.38(dt, 2H, J ) 5.34 Hz), 9.93 (d, 2H, J ) 5.51 Hz), 9.95 (d, 2H, J ) 5.99 Hz). 13C NMR (DMSO-d6), δ ppm: 48.28, 81.45, 123.31, 125.61, 126.30, 126.89, 127.75, 131.63, 134.90, 135.75, 136.47, 167.00. νCO 1721, 1687 cm-1; νNH 3100, 3051 cm-1. Anal. Calcd for RuC38H36N8O3P6F12‚ H2O: C, 42.98; H, 3.58; N, 10.55%. Found: C, 43.07; H, 3.51; N, 10.43. Instrumentation. Optical spectra were recorded on a Varian model Cary 500 NIR spectrophotometer, using 1 cm path length quartz cells. The FTIR spectra were measured in KBr pellets on a Bomem-Michelson 102 spectrometer in the 4000-400 cm-1 region. 1H and 13C NMR spectra were measured in a CD CN solution 3 using a BRUKER AC-200 system with tetramethylsilane as internal standard. CHN elemental analyses were performed on an EA 1110 CHNS-O Carlo Erba Instrument in the Microanalytical Laboratory at Universidade de Sa˜o Paulo, Sa˜o Carlos (SP). Cyclic voltammetry was performed on a Microquimica model MGPG-01 potentiostal. This instrument was also used for the characterization of the complexes. In the electrochemical experiments, the working and (9) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17, 3334 - 3341. (10) Ellis, C. D.; Margerum, L. D.; Murray, R. W.; Meyer, T. J. Inorg. Chem. 1983, 22, 1283-1291.
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Inglez et al. auxiliary electrodes were a stationary platinum foil and wire, respectively. The reference electrode was Ag/AgCl in a Luggin capillary. The measurements were taken at room temperature in freshly distilled CH3CN containing 0.1 mol L-1 of TBAPF6. Continuous Photolysis. Monochromatic irradiation at 400 nm was generated using a 200-W xenon lamp in an Oriel model 68805 universal arc lamp source selected with an appropriate interference filter (Oriel). The experiments were carried out at room temperature in 1.0 cm path length 4 side quartz cells capped with a rubber septum, and monitored by UV-vis spectrophotometry. The magnetically stirred solutions (∼10-4-10-3 mol L-1 initial complex concentration) were deoxygenated with dinitrogen. Potassium(trisoxalate)ferrate(III), used in actinometry at 450 nm, was prepared according to Calvert and Pitts.11 Luminescence and Quantum Yield Properties. Emission spectra at 25.0 °C were recorded on an Aminco-Bowman model J4-8960A spectrofluorometer, with a high-pressure xenon lamp and an IP 28 type photomultiplier. The corrected emission spectra were recorded using a Fluorolog 3 spectrofluorimeter from Ivon Jobin connected to an instrument setup file to correct the nonlinearity in the intensity of the excitation light and to adjust the detector sensitivity as a function of wavelength. The emission quantum yield was determined by comparison with a 10-4 mol L-1 acetonitrile solution of [Ru(bpy)3]2+ (φem ) 0.062) as a standard,12 using equation φem,S ) φem,R(AR/AS)(IS/IR)(nS/nR), where AS and AR are the absorption values of sample and reference, IS and IR are the integrated emission intensity of sample and reference, and nS and nR are the refractive indices of sample and reference.13 The rate constants for radiative (kr) and nonradiative (knr) decay are related to φem and τem by the following:14 kr ) φem/τem
(1)
knr ) (1 - φem)/τem
(2)
Quenching Experiments. Oxidative-quenching experiments of the excited complex were performed using N,N′-methyl-viologen chloride (methylviologen; MV+2) and [RuCl(NH3)5)]Cl2 as electron acceptor.14 At least 6 samples of the complex were prepared as aqueous solution, with MV2+ concentrations between 0 and 6 mmol L-1, and 0 and 9 mmol L-1 for [RuCl(NH3)5)]Cl2. Stern-Volmer plots14 of the observed decrease in the emission intensity with an increase in the quencher concentration showed a good linear relation with an intercept of 1, so that the oxidativequenching rate constant kq could be calculated using eq 3 and the emission lifetime of the complex in the absence of quencher, τem: I0/I ) 1 + kqτem[Q]
(3)
Luminescence Lifetimes and Kinetic Experiments. Timecorrelated single-photon counting (TCSPC) was used to determine the fluorescence lifetimes. This is a widely used technique and has been extensively reviewed.15 The apparatus comprised a frequencydoubled mode locked Nd:YAG laser (Coherent Antares 764) which synchronously pumped a cavity-dumped dye laser (Coherent 7013CD) with a medium gain of either Rhodamine 6G or DCM (4(11) Calvert, J. G.; Pitts, J. N. Photochemistry, 2nd ed.; Wiley: New York, 1967; pp 779-789. (12) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 55835590. (13) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620-6627. (14) Kavarnos, G. J.; Turro, J. N. Chem. ReV. 1986, 86, 401-449. (15) Hungerford, G.; Birch, D. J. S. Meas. Sci. Technol. 1996, 7, 121135.
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dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran). It produced a 3.8 MHz train of pulses with full width at halfmaximum (fwhm) of 6 ps and an average power of 1040 mW in the 300-800 nm range. The fluorescence emission was collected at 90°, and the emission wavelength was selected by a doublebeam monochromator. A Hamamatsu 1564-UO1 microchannel plate was used to detect the fluorescence emission, together with standard discriminators, time to amplitude converter (TAC), and a multichannel analyzer (Canberra 35), giving typical instrument response times of 50 ps (fwhm). All the decays were recorded to a minimum of 20000 counts in the channel of maximum intensity, and analyzed by iterative deconvolution and nonlinear least-squares fitting. The fits were validated by the reduced χ-squared, visually random residuals, and autocorrelation functions. Time-resolved optical (TRO) spectra were obtained using a laser flash-photolysis apparatus containing a Continuum Q-switched Nd: YAG laser (Continuum, Santa Clara, CA) with excitation provided by the third harmonic at λ ) 355 nm. The pulse length was 8 ns, the beam diameter incident on sample was 6 mm, and the repetition rate was 10 Hz. The laser pulse was set up from 15 mJ per pulse in the photobleaching studies measured with a Field Master power meter with L-30V head. The growth-decay kinetics were measured at a single wavelength using a monochromator (M300 from Bentham) and a photomultiplier (Hamamatsu, model R928P). Transient decays were averaged using a Tetronix TDS 340A digital oscilloscope. The stored digitized kinetic decays were transferred to a personal computer (PC) for the analysis with software supplied by Edinburgh Instruments. Theoretical Molecular Orbital Calculation. The calculations reported in this paper have been performed with the Gaussian 03 package.16 The starting molecular geometries were obtained at the UHF/3-21G level of theory. The final molecular geometry optimizations were performed using the Kohn-Sham density functional theory (DFT)17 with the Becke three-parameter hybrid exchangecorrelation function known as B3LYP18,19 alone with the pseudopotential basis set LanL2DZ.20 No symmetry condition was imposed. Vibrational frequencies were calculated from analytic second derivatives to check the minimum on the potential energy surface. The natural bond orbital calculation was performed using the NBO 5.0 program,21 as implemented in the Gaussian 03 package. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (17) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133. (18) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1998, B 37, 785-789. (19) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (20) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
Photoinduced Electron-Transfer Processes Scheme 1. Synthesis of 5,6-Bis(3-amidopyridine)-7-oxanorbornene Compound (1)
Scheme 2. Syntheses of cis-[Ru(bpy)2(3Amdpy2oxaNBE)](PF6)2 (2) and cis-[Ru(bpy)2(3Amnpy)2](PF6)2 (3) Complexes
In order to analyze the metal-ligand interaction energies and the composition of the orbitals, we used fragments 3Amdpy2oxaNBE, bipyridine, and atom Ru. This means that the MOs are expanded in the converged molecular or atomic orbitals of these fragments. The Mulliken population of a fragment orbital in a molecular orbital is used to denote the percentage of a fragment orbital character of that molecular orbital. As a first approximation to excitation energies, the differences between one-electron energies of the appropriate virtual and occupied molecular orbital may be used. (21) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Bohmann, Morales, C. M.; Weihold, F. NBO 5.0; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2001.
Results and Discussion Synthesis. The amine groups of 3-aminopyridine molecules were modified to amide functions via reaction with 7-oxanorbornene molecule modified by carbonyl functions, as illustrated in Scheme 1. The result is pyridine molecules connected via an amide-substituted-7-oxanorbornene unit (1). The pyridine units characterize a bidentate ligand which replaces the chloride ligands in the bis(bipyridine)-Ru(II) complex, as illustrated in Scheme 2, resulting in complex 2. Complex 3 with two unconnected pyridine units was prepared as a reference material,10 which is similar to complex 2 (Scheme 2). Complexes 2 and 3 differ by the different azo-groups in the pyridine units, where in complex Inorganic Chemistry, Vol. 46, No. 14, 2007
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Inglez et al.
Figure 1. Electronic absorption spectra in deaerated CH3CN solution.
Figure 2. Cyclic voltammograms in deaerated CH3CN solution ([Ru] ) 1 mmol L-1, 25.0 °C, 0.1 mol L-1 TBAPF6, scan rate 100 mV s-1, sensibility 50 µA).
2 an amide (amd) group is present while complex 3 has an amine (amn) group. Complexes 2 is very stable in air and exhibits great tolerance to the usual organic solvents for several hours. It was characterized using elemental analysis, IR, and 1H and 13 C NMR. Electronic Absorption Spectra. The absorption spectra of complexes cis-[RuCl2(bpy)2], 2, 3, and [Ru(bpy)3]2+ in CH3CN solution are depicted in Figure 1. The absorption maxima and extinction coefficients relevant to this investigation are summarized in Table 1 and agree with the literature.10,22 The absorption spectrum of 3 shows a band at 464 nm (max ) 8119 mol-1 L cm-1) with a shoulder around 431 nm, whereas the spectrum of 2 shows a band at 460 nm (max ) 10247 mol-1 L cm-1) with a shoulder around 498 nm. Separate charge-transfer bands have been observed in a series of mixed-ligand complexes.6,23 The lowest energy (22) Fergusso, J. E.; Harris, G. M. J. Chem. Soc. A 1966, 1293-1296. (23) (a) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617-1622. (b) Barigelletti, F.; Juris, A.; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1983, 22, 3335-3339.
5748 Inorganic Chemistry, Vol. 46, No. 14, 2007
Figure 3. Electronic spectral changes resulting from the 440 nm photolysis of complex 3 (0.12 mmol.L-1) in deaerated CH3CN solution.
absorptions usually correspond to promotion of an electron to the most easily reduced ligand. Figure 2 shows that, for [RuCl2(bpy)2], 2, 3, and [Ru(bpy)3]2+, oxidations occur at +0.33, 0.81, 1.16, and 1.34, and reductions occur at + 0.25, 0.73, 1.06, and 1.23 versus Ag/AgCl electrode, respectively. Thus, in 2, the absorption at 460 nm is assigned to the MLCT processes involving the bpy ligand, and the absorption at 498 nm belongs to CT transitions from Ru dπorbitals to the lowest π* orbital of the 3Amdpy2oxaNBE ligand. The differences in potential between the oxidations and reductions are related to the energy required to excite photochemically an electron from a metal dπ into a ligand π*.14,24 Figure 1 shows that attaching the 7-oxanorbornene molecule to the 3Amnpy ligand also has an influence on the position and relative intensity of the absorption band at 350 nm, which shifts to a longer wavelength and becomes weaker going from 2 to 3. This spectroscopic behavior was observed earlier for [Ru(bpy)2LL′]2+ based complexes with chelating ligands.25 Continuous Photolysis. The photolysis of complex 3 (0.12 mmol L-1 in CH3CN) with 440 nm light led to a decrease of the characteristic absorption band at 464 nm (Figure 3) and the appearance of a new band at 426 nm ((max ) ca. 7600 mol-1 L cm-1). The spectral changes at 446 and 434 nm suggest the occurrence of successive photoreactions. The MLCT absorption bands of the related solvento cis[Ru(bpy)2(pyridine)(CH3CN)]2+ (λmax ) 440 nm; max ) 9700 mol-1 L cm-1) and cis-[Ru(bpy)2(CH3CN)2]2+ (λmax ) 426 nm; max ) 8700 mol-1 L cm-1) complexes occur in the same spectral ranges observed for the photoproducts in the present case.26 Thus, a possible explanation for the spectral changes would be a simple 3Amnpy photosolvation to form cis-[Ru(24) Yang, X. J.; Janiak, C.; Heinze, J.; Drepper, P. M.; Piotrowski, H.; Klufers, P. Inorg. Chim. Acta 2001, 318, 106-116. (25) Root, M. J.; Sullivan, B. P.; Meyer, T. J.; Deutsch, E. Inorg. Chem. 1985, 24, 2731-2739. (26) (a) Durham, B.; Walsh, J. L.; Carter, C. L.; Meyer, T. J. Inorg. Chem. 1980, 19, 860-865. (b) Pinnick, D. V.; Durham, B. Inorg. Chem. 1984, 23, 1440-1445. (27) (a) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous Solution; Butterworths: London, 1972. (b) Guven, A. Int. J. Mol. Sci. 2005, 6, 257-275.
Photoinduced Electron-Transfer Processes Table 1. Electronic Transitions of Ru(II) Complexes in CH3CN at Room Temperaturea λ, nm (, mol-1 L cm-1) complex
LC; π f π*
[Ru(bpy)3]2+
238 (30000) 250 (25000) 286 (87000) 240 (23750) 294 (90000) 242 (31807) 292 (42055) 242 (40836) 292 (90000)
[RuCl2(bpy)2] complex 3 complex 2 a
MLCT; dπ f π*
LMCT; π f dπ
323sh (6900) 345 (4500)
420 (8035) 451 (9800)
375 (9703)
475sh (5100)
524 (7887)
328 (14768)
431sh (7220)
464 (8119)
342 (16367)
460 (10247)
498 (9762)
551 (9703)
sh ) shoulder.
Table 2. Photophysical Properties of Complexes 2 and 3 and Related Bipyridine Ru(II) Complexes complex
solvent
[RuCl2(bpy)2] [Ru(bpy)3]2+ complex 3 complex 2
CH3CN CH3CN CH3CN CH3CN H 2O
λ
τem (ns)
no luminescence 598 870 no luminescence 594 650 591 509
φem
∆E (abs - em)
10-4 kr (s-1)
10-6 knr (s-1)
Ea (cm-1)
0.061
5400
7.1
1.08
850
0.0015 0.0012
5100 6000
0.23 0.23
1.54 1.96
987
(bpy)2(3Amnpy)(CH3CN)]2+ and cis-[Ru(bpy)2(CH3CN)2]2+ in a stepwise fashion under continuous photolysis. This pattern suggests that the 3Amnpy labilization may result from the population of a dissociative excited state or an excited state with a relatively low barrier for dissociation. A likely candidate would be the mixed d-π*/d-d state. In addition, the 3Amnpy ligand is a relatively weak-field ligand.28 Therefore, it might be labile in metal complexes with low-energy MC excited states, as in the case of Ru(II) hexacoordinate complexes.26 In contrast, complex 2 appers to be photochemically unreactive. No detectable UV-vis spectral changes could be observed after 2 h. The lack of photoreaction in solution for complex 2 is a consequence of the chelate ring of 1, which contributes to the stability of the complex 2 under continuous light irradiation. Luminescence Investigations. The emission maxima (λmax, em), quantum yield (φem), and lifetimes (τem) measured for the photostable complex 2 and related complexes28 are listed in Table 2 and Figures 4 and 5. The emission band of complex 2 is broad and structureless, even in sucrose film, as illustrated in Figure 5. Figure 4 and Table 2 show that complex 2 exhibits a large Stokes shift similar to that observed for analogous complexes.29 Whereas the emission spectra of complex 2 were independent of the excitation wavelength, between 260 and 550 nm, under the same circumstances the related complex 3 shows no luminescence. These results indicate that the emission originates from the lowest excited state, which can be populated by either a direct optical excitation or an efficient internal conversion from higher charge transfer states. (28) (a) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444-2453. (b) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem. Soc 1982, 104, 4803-4810. (c) Watts, R. J. J. Chem. Educ. 1983, 60, 834-842. (29) (a) Villegas, J. M.; Stoyanov, S. R.; Rillema, D. P. Inorg. Chem. 2002, 41, 6688-6694. (b) Tyson, D. S.; Luman, C. R.; Zhou, X. L.; Castellano, F. N. Inorg. Chem. 2001, 40, 4063-4071.
A comparison between the absorption and the emission spectra of [Ru(bpy)3]2+ and 2 is of interest here, since the same orbital will be involved in the absorption and emission of these complexes. Figure 6 shows the absorption spectrum of [Ru(bpy)3]2+ and the emission and excitation spectrum of 2. The excitation spectrum of 2 measured by monitoring the emission at 590 nm matches perfectly the absorption spectrum of [Ru(bpy)3]2+, indicating that the emission of 2 around 600 nm possesses an MLCT character from the bypiridine ligand.30 It is interesting to note that replacing bpy by 3Amdpy2oxaNBE in the [Ru(bpy)3]2+ causes a decrease in the emission lifetime; however, the Stokes shifts are unaffected (Tables 1 and 2). As shown in Figure 7, the emission of complex 2 in CH3CN increases in intensity upon lowering the temperature, and the emission quantum yield increased with decreasing temperature. At the lowest temperature studied (77 K), φem ) 0.22, while at room temperature is φem ) 0.0015. Table 2 shows the lifetime decay for complex 2 in deaerated H2O and CH3CN solutions at room temperature. The τem versus T data could be fitted by eq 1/τem ) k′ + A exp(-Ea/RT)28,31 with correlation coefficient of 0.975 (Figure 7, insert). The kr, knr, and Ea values are summarized in Table 2. On the basis of the model proposed for [Ru(bpy)3]2+ and developed by Crosby,31 the lowest-energy MLCT triplet is coupled to the ground state and to one higher-lying triplet state. Crosby31 suggested that emission occurs from a cluster (30) One of the reviewers made the following comment: “the interpretation of Figure 6, may be correct, but the presence of a [Ru(bpy)3]2+ impurity participating in the emission can not be ruled out”. The reviewer argues “[Ru(bpy)3]2+ compounds are very stable and favored, and tend to form as an impurity in the synthesis of [RuCl2(bpy)2]2+ from RuCl3, even under careful stochiometric control”. However, our deduction that the emission showed in Figure 6 is related to the presence of compound 2 in solution is mantained by analogy with both emission quantum yield and lifetime values that are smaller than those values found for [Ru(bpy)3]2+ (see Table 2). (31) (a) Hager, G. D.; Crosby, G. A. J. Am. Chem. Soc. 1975, 97, 70317037. (b) Hager, G. D.; Watts, R. J.; Crosby, G. A. J. Am. Chem. Soc. 1975, 97, 7042-7048.
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Inglez et al.
Figure 4. CH3CN.
Absorption and emission spectra of complex 2 in
Figure 5. Emission spectra of complex 2 in sucrose film (a), CH3CN (b), and H2O (c) at room temperature.
Figure 6. Normalized absorption spectrum of [Ru(bpy)3]2+ (a) and normalized emission (b; λexc) 450 nm) and excitation (c; λem ) 590) spectra of complex 2 in CH3CN.
of closely spaced levels (∆E ) 100 cm-1), having similar but not identical properties. It is noteworthy that for complex 2 the access to this triplet state involves crossing a barrier of 987 cm-1. According to previous works,31 the uppermost triplet state is assumed to be an MC state strongly coupled to the ground state.
5750 Inorganic Chemistry, Vol. 46, No. 14, 2007
Figure 7. Emission decrease of complex 2 when increasing the temperature (10.0, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 45.0, and 50.0 °C) in CH3CN. Insert: Arrhenius plot for τ data.
Though complex 2 is sufficiently photoinert at ambient temperature for exploitation as photosensitizer of photoredox and energy transfer processes, related complex 3 is photoreactive, and the presence of nearby MC states is responsible for such photolability. The emission lifetime is mainly determined by the rate constant of the nonradiative decay, knr, which decreases by a factor of 31 on going from bpy to 3Amdpy2oxaNBE in [Ru(bpy)3]2+. This order reflects the decreasing Ru f bpy π-back-donation. Many factors can effectively contribute to the nonradiative deactivation of the MLCT excited state of 2, such as the increasing mixing of dπ(Ru) and π*(bpy)/π*(3Amdpy2oxaNBE) orbitals. Also, the increased flexibility of the 3AmdpyoxaNBE ligand and the extensive delocalization within the Ru-(3Amdpy2oxaNBE) chelate ring may provide efficient nonradiative deactivation pathways. The lifetime in CH3CN or H2O solution suggested that complex 2 should be well suited for the investigation of photoinduced redox processes in large molecular assemblies. Quenching Reactions. In order to understand the chemical properties of complex 2 in its excited state, the oxidative emission quenching by methylviologen (MV2+) and [RuCl(NH3)5]2+ ions were investigated. These quenchers afford linear Stern-Volmer plots (φ0/φ vs quencher concentration),32 indicating the bimolecular character of the quenching reactions. The bimolecular rate constants, kq, were calculated from the slopes kSV of the Stern-Volmer plots (kq ) ksv/τ0; τ0 ) 509 ns). The values of kq were lower than those controlled by diffusion (1.3 × 108 for MV2+ and 1.2 × 108 mol-1 L s-1 for [RuCl(NH3)5] 2+ ), and lower than previously estimated for [Ru(bpy)3]2+ (2.4 × 109 mol-1 L s-1 for MV2+).33 Laser Flash Photolysis Experiments. Figure 8 shows spectral and kinetic results for complex 2 in water after (32) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815-4824. (33) Mahon, C.; Reynolds, W. L. Inorg. Chem 1967, 6, 1927-1928.
Photoinduced Electron-Transfer Processes
Figure 9. Decays for complex 2 in H2O (1.5 × 10-5mol-1 L) recorded at 390 nm in the absence (a) and presence (b) of MV2+; 7.4 mmol L-1.
excitation with an 8 ns pulse at 355 nm. The kinetics was monitored at 390 and 480 nm, and the difference spectrum was recorded 10 ns after photoexcitation. The transient absorbance measurements of 2 (27 µ mol L-1) led to strong transient bleaching between 350 and 480 nm. Although at 390 nm the transient exhibited a large ∆OD, for both wavelengths studied (390 and 480 nm), the initial bleach decayed to the original baseline. The ground-state repopulation could be fitted to simple exponential decay kobs ) 5.3 ( 0.9 × 107 s-1 at 390 nm and 8.3 ( 0.9 × 107 s-1 at 480 nm. Studies of [Ru(bpy)3]2+ complex have shown that the MLCT excited-state spectrum can be modeled in terms of {MIII bpy•-} chromophore34 with bpy•- intraligand transitions providing a distinctive signature. Complex 2 (Figure 8) strongly resembles *[Ru(bpy)3]2+ (the MLCT state of [Ru(bpy)3]2+), especially in the intense ∼390 nm band. Thus, the expected MLCT character of the nanosecond excited state of complex 2 is supported by the nanosecond spectral data. When such studies were performed in aqueous solutions equilibrated with MV2+ (1.6-7.4 mmol L-1), Figure 9, the magnitude of long-lived absorbance change measured at 390
nm decreased from the one seen in the absence of quencher, resulting in kobs ) 1.0 ( 0.2 × 108 s-1. Furthermore, Figure 9 also shows that the initial bleach evolved into a transient absorbance. Theoretical Analysis. As the nature of the excited states that account for the photochemical properties of complex 2 depends on the character of the MOs involved in the excitations, a comprehensive description of the ground state MO composition of this complex is given. Bond distances and bond angles selected from the optimization of this molecule using two DFT/basis set combinations are listed in Table 3. For comparison, the X-ray results for 4-aminopyridine (4Amnpy) related complex structure are also listed.35 The structure of 2 is presented in Figure 10. In general, the DFT procedures gave optimized structures where both Ru-N(bpy) and Ru-N(3Amdpy2oxaNBE) distances agreed with the experimental data for the cis-[Ru(bpy)2(4Amnpy)2](ClO4)2 complex. Moreover, the asymmetrical carbonyl array justifies the multisignals in the IR and NMR spectra. Complex 2 shows one bpy molecule, one pyridine ring N atom of the other bpy, one pyridine ring N atom of 3Amdpy2oxaNBE, and the Ru atom lying in the same plane. The other N-pyridine arm of 3Amdpy2oxaNBE is trans-positioned to a pyridine unit of a bpy localyzed in the z axis. The coordination around the central Ru atom is nearly octahedral. The two N-pyridine atoms of the 3Amdpy2oxaNBE form angles of ca. 172° with each one of the N-pyridine atoms of the bpy ligands that are trans-positioned. Whereas the mean interatomic distances Ru-N(bpy) are not sensitive to the substitution of 4Amnpy for 3Amdpy2oxaNBE, the Ru-N(3Amdpy2oxaNBE) distance based on the DFT calculation (2.2 Å) is somewhat longer than that found in cis-[Ru(bpy)2(4Amnpy)2]2+ based on the X-ray data (2.12 Å).35 This may be attributed to the π-acceptor nature of the amide-substituted pyridine units in 1 increasing the π-electron competition. Furthermore, the angle (3Amdpy2oxaNBE)N-Ru-N(3Amdpy2oxaNBE), 96.5°, is larger than
(34) Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N. J. Am. Chem. Soc. 1980, 102, 1309-1319.
(35) Chun-Ying, D.; Zhong-Lin, L.; Xiao-Zeng, Y, Mak, T. C. W. J. Coord. Chem. 1999, 46, 301-312.
Figure 8. Transient difference spectra and kinetics for complex 2 in H2O (1.5 × 10-5 mol-1 L). Upper: Difference spectra recorded 10 ns after 355 nm laser excitation. Lower: Decays recorded at 390 (a) and 480 (b) nm.
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Inglez et al. Table 3. Optimized Bond Lengths and Bond Angles in the Electronic Ground State of Complex 2 and the Comparison with X-ray Data for Related Complex optimized geometry of complex 2 X-ray data of cis-[Ru(bpy)2(4Amnpy)2](ClO)2a Ru-eqN(bpy; trans Apy) Ru-eqN(bpy) Ru-axN(mer bpy) Ru-eqN(mer bpy) Ru-axN(Apy) Ru-eqN(Apy) C(py ring)-N(amine or amide; ax Apy) C(py ring)-N(amine or amide; eq Apy) C(amide)-N(amide; ax Apy) C(amide)-N(amide; eq Apy) N-Ru-N (eq bpy bite angle) N-Ru-N(mer bpy bite angle) N-Ru-N (ax/eq Apy’s expected bite angle) eqN(bpy)-Ru-eqN(bpy) axN(Apy)-Ru-axN(bpy) eqN(Apy)-Ru-eqN(bpy)
Bond Lengths (in Å) 2.060(1) 2.070(1) 2.076(1) 2.057(1) 2.104(1) 2.121(1) 1.367(1) 1.366(1)
Bond Angles (deg) 80.0(1) 78.7(1) 91.7(1) 173.3(1) 175.8(1) 176.0(1)
3-21G 2.091 2.112 2.100 2.107 2.177 2.198 1.428 1.425 1.385 1.372 78.4 78.0 97.4 175.9 170.7 171.1
LANL2DZ 2.093 2.107 2.085 2.102 2.208 2.177 1.432 1.425 1.391 1.389 78.3 78.6 96.5 175.8 173.4 172.0
a Reference 34; eq ) equatorial (one or two py N’s on xy plane); ax ) axial (py N on z axe); mer ) meridional (two py N’s on yz or xz plane); Apy ) 3Amdpy unit in 1 or 4Amnpy.
Figure 10. Final molecular geometry of complex 2 optimized by using density functional theory (DFT).
the one found for N(bpy)-Ru-N(bpy), 78°, reflecting the large hole size of the 3Amdpy2oxaNBE ligand. The large bite ring could explain the reason why the two py rings of 3Amdpy2oxaNBE are not in the same plane, contrary to the bpy molecule. This lack of symmetry is experimentally supported by the increasing absorption at ca. 350 nm, going from [Ru(bpy)3]2+ to complex 2 (Table 1; Figure 1). Molecular Orbital Analysis. The MO composition of complex 2 is given in Table 4. The LUMO and the two following empty MOs, LUMO + 1 and LUMO + 2, have 100% 3Amdpy2oxaNBE π* and more than 90% bpy π* character, respectively. In the same way, instead of a single HOMO, there are three close-lying highest occupied orbitals, which give rise to degenerated orbitals in the related complex36 [Ru(bpy)3]2+. Those MOs are a combination of bpy π and ruthenium d orbitals, which have an antibonding (36) Daul, C.; Baerends, E. V.; Vernooijs, P. Inorg. Chem. 1994, 33, 35383543.
5752 Inorganic Chemistry, Vol. 46, No. 14, 2007
π interaction. Their energies are ∼3 eV below the set of π* orbitals of 3Amdpy2oxaNBE and bpy ligands. Therefore, besides d-d (Ru) and MLCT (Ru f 3Amdpy2oxaNBE) bands, we can also expect MLCT (Ru f bpy) and LLCT (3Amdpy2oxaNBE f bpy) transitions. In the previous section, the lowest energy absorptions bands of complex 2 at 498 (2.5 eV) and 460 (2.7 eV) nm were assigned as a contribution from both metal to ligand (dπ f 3Amdpy2oxaNBE) and (dπ f bpy) charge-transfer MLCT transitions, respectively. This assignment was based on the high intensity of the band, its solvatochromism (H2O, λabs ) 468; CH2Cl2, λabs ) 503 nm), ligand dependence of its position, and comparison with related complexes: see Table 2. According to the MO diagram (Table 4), the main transitions of the first and second absorption bands originate from the highest filled metal dπ orbitals to the π* orbital of 3Amdpy2oxaNBE (3.2 eV) and bpy (3.4 eV), respectively. Both assignments are in agreement with the CT character of these absorption bands. The above results show that the two homologous complexes 2 and 3 undergo different primary photoprocesses upon irradiation into their lowest-energy absorption bands. Complex 3 loses 3Amnpy, and 2 shows a strong luminescence similar to the one found for [Ru(bpy)3]2+, which is a remarkable photoprocess for a pyridine based complex. The occurrence of a 3Amnpy-loss reaction is expected, since a similar reaction has been observed for related Ru complexes, such as cis-[Ru(bpy)2L2] (L ) py; and substituted pyridines).26 For these complexes, the efficiency of the photoreaction appeared to depend on the substitution of pyridine and on the energy difference between the MLCT and MC states.26 The photoreaction was therefore proposed to proceed from the lowest MC state, after surface crossing from the MLCT states.26 This is associated with the chelating nature of the 3Amdpy2oxaNBE ligand present in 2. A similar
Photoinduced Electron-Transfer Processes Table 4. Characters, Energies, and Countour Plots of LUMOs and HOMOs of Complex 2 Calculated by DFT
mechanism may explain the photochemistry of 3. Concerning complex 2, the first absorption band shifts to shorter wavelengths, and the lowest excited state decreases in energy with respect to the relative MC states and approximates its energy to the dπ f bpy CT transition. As a result, complex 2 does not show a release of the 3Amdpy moiety, but it shows luminescence similar to the one found in [Ru(bpy)3]2+. For the interpretation of these effects, we have to consider the character of the orbitals involved in the electronic transitions and the nature of the lowest excited states of these complexes. The emissive lowest-lying excited state in the [Ru(bpy)3]2+ complex originates from electron excitation of the dπ antibonding orbital of the metal to the π*-orbital of the bpy ligand. The thermally activated decay of the excited state of [Ru(bpy)3]2+ has been ascribed to a thermal population of a reactive d-d state.28 Due to the coordination of the relative weak-field 3Amnpy ligand, the empty d-orbitals of the Ru-3Amdpy2oxaNBE complex are expected to lie lower in energy than those of [Ru(bpy)3]2+. Therefore, the MC states are expected to occur at lower energy, thermally accessible from the lowest CT state, explaining the photo-
chemical activity of the bis(3Amnpy) complex. The temperature dependence of the excited-state lifetime may be better explained by deactivation through a close-lying higher charge transfer state. Conclusions The complex 2 ion constitutes a new class of emissive transition metal complex with long-lived excited states of a charge-transfer character. The oxidative quenching experiments show that this complex may be considered an electrontransfer sensitizer and/or catalyst in photochemical reductions and it could be used as a building block for the construction of photochemically active redox molecular devices. Acknowledgment. The authors would like to acknowledge the financial support from FAPESP, CAPES, and CNPq. They are also indebted to Dr. Amando Siuiti Ito from FFCLRP-USP for supplying the equipment for fluorescence lifetime measurements and Angela Cristina Pregnolato Giampedro for reading this manuscript. IC062478V
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